Polymers having an olefinically unsaturated double bond are advantageously used in vulcanization because of the unsaturated double bond, but the double bond also causes poor stability in resistance against heat or oxidation. The poor stability of these polymers can be markedly improved by hydrogenation to remove the unsaturated double bonds in the polymer chain. However, compared with low-molecular weight compounds, polymers are less susceptible to hydrogenation due to the influences of the viscosity of the reaction system, the steric hindrance of the polymer chain, etc. Moreover, after completion of hydrogenation, it is extremely difficult to physically remove the spent catalyst. Complete separation of the catalyst is practically impossible. Therefore, in order to achieve hydrogenation of polymers with economical advantages, it has been keenly demanded to develop a catalyst for hydrogenation which exhibits sufficient activity in such a low amount that it needs no removal, or a catalyst for hydrogenation which can be easily removed. In addition, if an alkali metal compound remains in a hydrogenated polymer in a high content, the weathering discoloration characteristic of the polymer is deteriorated.
The inventors of the present invention previously discovered a process for hydrogenating an olefin compound using a combination of a specific titanocene compound and an alkyl lithium (JP-A-61-33132 and JP-A-1-53851, the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a process for hydrogenating an olefinically unsaturated (co)polymer by using a combination of a metallocene compound, an organoaluminum compound, zinc, and magnesium (JP-A-61-28507 and JP-A-62-209103), and a process for hydrogenating a living polymer containing an olefinically unsaturated group by using a combination of a specific titanocene compound and an alkyl lithium (JP-A-61-47706 and JP-A-63-5402). However, these catalyst systems, while being highly active, are difficult to handle and have unsatisfactory storage stability for a long period of time. Further, these systems require a reducing agent (organometal) to reduce a titanocene compound to a lower-valent compound, and the hydrogenation activity largely varies depending on the molar ratio of the reducing agent to the titanocene compound and on the contacting conditions, which also makes the catalyst system difficult to handle.
Hydrogenation of the double bond of an olefinically unsaturated double bond-containing polymer using a reaction product between a Tebbe's reagent (a metallacycle compound of a titanocene compound and trimethylaluminum) and an alkyl alkali metal compound is known (U.S. Pat. No. 5,244,980). In this process, it is disadvantageous that a large amount of an alkyl alkali metal compound is required as a cocatalyst to achieve high hydrogenation activity. In addition, because the catalyst used in this process has poor catalyst activity, a large amount of the catalyst is needed.